Iran has said that operations have begun at two uranium mines and a milling plant in Iran.
It said that Western opposition would not slow its nuclear work, days after talks with world powers made no breakthrough.
Iran opened the Saghand 1 and 2 mines in the central province of Yazd.
The Shahid Rezaeinejad yellow cake plant is in the town of Ardakan in the same region to mark the country's National Nuclear Technology Day.
Yellow cake can be further processed into enriched uranium to make fuel for nuclear power plants.
Also yellow cake can be used to provide material for atomic bombs if refined much more, which the West fears may be Iran's ultimate goal.
Talks between Iran and six world powers held in Kazakhstan last week failed to make progress in resolving a decade-old dispute that threatens to trigger a new war in the Middle East.
The powers want Iran to curb its uranium enrichment activity.
Iran denies Western accusations that it is seeking an atomic weapons capability.
It often announces technical advances in its nuclear programme, but these can be difficult to verify independently.
Iran has for years carried out construction work at Saghand and Ardakan.
